Thursday 17 July 2014

Three things would be Very Careful While lifting Anchors. Origin, Magnitude and Distribution of Load


Lifting anchors are loaded by the self weight of the concrete element, by forces due to adhesion and form friction and by dynamic actions due to acceleration.
The self weight of the precast concrete element can be calculated using a specific gravity of 23 to 25 kN/m3 for normal weight concrete, depending on the percentage of reinforcement. Mostly this value is already known from the static calculation.

Inertia forces act when the element is accelerated or decelerated by the lifting equipment. These forces must also be transferred by the lifting anchors. The size of the inertia forces scatters between 5 and 50 % of the elements weight depending on the type of crane and the ratio between the weight of the element and the capacity of the crane. Under normal conditions it should be conservative to calculate inertia forces with 30 % of the self weight . Additional considerations have to be made for special situations like transport of elements through rough terrain, for example with an excavator. Inertia forces up to 300 % of the self weight can be expected in such cases.
When the concrete element is lifted out of the mould, forces between the concrete and the formwork surface, due to adhesion and form friction must be added to the self weight. As long as the formwork is fixed to the floor or heavy enough to stay in place (it normally is), dynamic actions must not be considered together with adhesion and form friction. Only in cases where the form is not fixed and not heavy enough to stay in place, inertia forces have to be taken into account on the mass of the element and on the mass of the formwork as well. Later, when the element is moved in the precast plant or on site, self weight and inertia forces both have to be considered.
After the estimation of the forces acting on the element, forces on each anchor have to be calculated, taking into account the position of anchors, number and length of ropes or chains and the static system. In most cases, the aim is to have a statically determinate system, because then the forces on each anchor and on each rope or chain can be clearly calculated. In statically indeterminate systems, load distribution depends on length and stiffness of the ropes, which are mostly unknown. If statically indeterminate systems are used for special reasons, only the statically determinate part should be used in calculating the load distribution, and additional ropes or chains should only be used for stabilization.

Thursday 10 July 2014

Design Basis for (FRP) Fiberglass Reinforcement Concrete


A design basis for FRP reinforced concrete has been recommended by a number of standards and professional organization. The LRFD basis is recommended by the ACI 440,1R-03; at this time resistance factors are not probabilistically based. ;load factors are those recommended for all concrete structures by AC! 318-95 (1995)
For the design of flexural members reinforced with FRP rebars the ACI 440.1R-03 recommends the following resistance factors:
Flexural capacity (tensile reinforcement only):
ø= 0.5 for an under reinforced beam section (pf< pfb)
ø=0.7 for a substantially over reinforced beam section (pf> 1.4Pfb)
ø= 0.5pf/pfb, for a lightly over reinforced beam section (pfb< p < 1.4pfb)
Shear capacity (stirrups):
ø =0.85 per ACI 318-95.
where Pf is the FRP reinforcement ratio and Pfb is the balanced FRP reinforcement ratio.
Characteristic strength (also called the guaranteed strength) and strain to failure of FRP rebar’s are defined as the mean minus three standard deviations of a minimum of 25 test samples. The design strength, fro, and design failure strain, Efu, are obtained from the characteristic strength and failure strain by multiplying them by an environmental decline factor, CE, which relies on the fiber type in the bar and the type of intended service of the structure. For example, for weather exposed concrete with glass FRP rebar’s, CE is 0.7 (ACI 440.1R-03 2003).
Since FRP rebar’s typically have a lower modulus than steel rebar’s, the serviceability limit state (deflections and crack widths) can often control the design of FRP reinforced concrete sections. The ACI 440.1 R-03 provides procedures for calculating deflections and crack widths in FRP reinforced members.